In recent years, displays in image display devices, such as television devices, are being shifted from conventional cathode-ray tube displays to thin displays, such as liquid crystal displays and plasma displays. With the thin displays, thicknesses of the image display devices can be decreased. Because liquid crystal panels do not emit light, liquid crystal display devices including liquid crystal panels require backlight devices. The backlight devices are classified broadly into a direct type and an edge-light type based on mechanisms. For further reduction in thicknesses of the liquid crystal display devices, the edge-light type backlight devices including light guide plates are more preferable.
An edge-light backlight device may include an optical sheet that includes lenses and geometric structures on a light emitting side. Each lens is formed independently in a half-dome-like shape. The geometric structures continue from one another. The optical sheet is configured to control the brightness within a surface thereof by adjusting in-plane ratios of the half-dome-like lenses having high light diffusivity and the geometric structures having high light collecting capability. The optical sheet is arranged over alight exit surface of a light guide plate that is configured to guide light from a light source.
Because the backlight device requires the optical sheet having a special configuration other than the light guide plate, parts cost of the backlight device is high. Furthermore, light from the light guide plate passes through the optical sheet having the special configuration. Therefore, a length of an optical path until the light reaches a liquid crystal panel may be long and thus light use efficiency may decrease.
The present invention was made in view of the above reasons and an object of this invention is to improve evenness of brightness distribution without using a special component.